An audio system includes an output transducer, such as a loudspeaker, to produce a sound pressure wave in response to an input signal representative of a desired sound pressure wave. Most loudspeakers, however, generate an actual sound pressure wave that differs from the desired sound pressure wave represented by the input signal. This difference is due, in part, to non-linear aspects of the loudspeaker. In particular, the diaphragm of a loudspeaker has a non-linear stress-strain curve. Furthermore, the motion of the diaphragm results in the delay modulation of higher frequencies by lower frequencies. Accordingly, there have been efforts in the art to compensate for these and other factors which cause a loudspeaker to produce an actual sound pressure wave which is different from the desired sound pressure wave.
For example, U.S. Pat. Nos. 4,426,552 and 4,340,778 both to Cowans et al. and both entitled "Speaker Distortion Compensator," disclose means coupled to each speaker in a system for compensating for mass, compliance, and damping. The processing circuits are exemplified by active and passive circuits which provide a feedforward component which nullifies the spurious emanations that would otherwise develop as the loudspeaker diaphragm attempts to follow complex motions that are otherwise impermissible because of its dynamics.
U.S. Pat. No. 4,709,391 to Kaiser et al. entitled "Arrangement For Converting An Electric Signal Into An Acoustic Signal Or Vice Versa And A Non-Linear Network For Use In The Arrangement" discloses an arrangement including means for reducing distortion in the output signal. The reducing means comprise a non-linear network arranged for reducing non-linear distortion by compensating for at least a second or higher order distortion component in the output signal.
Furthermore, the article by de Vries et al. entitled "Digital Compensation of Nonlinear Distortion in Loudspeakers," IEEE, 1993, pp. I-165 to I-167, discloses a method to compensate for non-linear distortions produced by a loudspeaker in real-time by non-linear digital signal processing. An electrical equivalent circuit of an electrodynamic loudspeaker is developed resulting in a linear lumped parameter model. The linear model is extended to include non-linear effects, and an inverse circuit is implemented in real-time on a digital signal processor.
Notwithstanding the above mentioned references, there continues to exist a need in the art for improved audio systems and methods which compensate for the non-linear aspects of a loudspeaker. This need is critical in telephony and particularly in speakerphone applications where a small loudspeaker is used. This need is even more critical in cellular speakerphone applications where intelligibility is difficult to begin with.